Experiments were carried out to analyze functional interactions between early viral proteins of oncogenic DNA viruses from different groups. The SV40 A gene product (T antigen) could complement a mutant of human adenovirus temperature sensitive DNA replication in semi-permissive monkey cells at the non-permissive temperature. Early proteins from adenovirus (presumably by 72 Kd early Ad 2 protein) could regulate the over production of SV40 early mRNA synthesis by tsA mutant of SV40. Low stringency hybridization experiments indicated that limited DNA sequence homology occurred between SV40 and adenovirus at control regions for replication and transcription, where the SV40 T antigen and adenouvirus 72 Kd protein bind. We have also found a novel modification, poly ADP-ribosylation, present in a minor population of T-antigen. The absence or presence of this modification may be important for replication. These findings suggest that there can be mutual interaction of viral proteins between viruses of different groups which function at the level of replication and transcription. Future work includes